I live on a hill where the soil has lots of clay. My back garden has been terraced with small (1m) retaining walls. The net result is that we have several lawns / gardens with the following properties:
- Surrounded on all sides by concrete retaining walls.
- The top 20-30cm is good soil; the remainder is clay.
- At the bottom of some of the retaining walls (i.e. a bit less than 1m below the surface), there are periodic drainage holes.
So the drainage is quite terrible -- the water can't get out the sides, and it can only sink slowly through the clay. To improve this, my idea was to dig a trench ~30cm wide along the side of the retaining walls (where the drainage holes are), fill the bottom 50cm with sand, and the rest with dirt, and then run drainage pipes from the rest of the lawn to this trench.
On the other hand, I have also read: "Don't mix sand and clay; that's how you get brick". Is this a good idea? Will it work? Are there better options?
Edit: Photos below (excuse the laundry -- we can't let a sunny day go to waste :-) )
The corner with the manuka (centre first image, bottom-right second image) is evidently a little lower than the rest of that lawn, and thus in heavy rain it can be under 5+ cm of water.